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a. Infiltration facilities shall only be allowed if the soils of the facility fall within <br />hydrologic soil groups A or B, and if the seasonal high water table and any <br />underlying bedrock are at least six feet below the final grade elevation. <br />b. All runoff directed into an infiltration basin must first flow through an extended <br />conveyance facility to remove coarser sediments that could cause a loss of <br />infiltration capacity. <br />c. During construction, all runoff from disturbed areas of the site shall be diverted <br />away from the proposed infiltration basin site. No construction equipment shall <br />be allowed within the infiltration basin site to avoid soil compaction. <br />4. Additional criteria applyin~ to extended conveyance facilities. <br />a. Facilities shall be lined with fine turf-forming, flood tolerant grasses. <br />b. Facilities designed according to the extended conveyance detention design drain <br />time shall: <br />(1) Not be located in areas where the depth to bedrock and/or seasonal high <br />water table is less than 3 feet below the final grade elevation. <br />(2) Only be allowed where the underlying soil consists of hydrologic soil <br />group (HSG) A or B, unless the underlying soil is replaced by at least a <br />2.5 foot deep layer of soil amendment with a permeability equivalent to a <br />HSG A or B soil .and an underdrain system is provided. <br />Facilities designed according to the flow through design drain time shall: <br />(1) Only be allowed on sites where the total area disturbed is 5 acres or less. <br />(2) Be designed to slow and filter runoff flowing through the turf grasses <br />with a maximum depth of flow no greater than 3 inches. <br />d. Concentrated runoff shall be converted to sheet flow before entering an extended <br />conveyance facility designed according to the flow through drain time. <br />5. Additional criteria for extended detention facilities: <br />a. The outlet shall. be designed to release the bottom 50 percent of the water quality <br />volume in no less than 2/3rd of the drain time. A valve shall be provided to drain <br />any permanent pool volume for removal of accumulated sediments. The outlet <br />shall be designed to minimize clogging, vandalism, and maintenance. <br />b. The basin design shall incorporate the following features to maximize multiple <br />uses, aesthetics, safety, and maintainability: <br />(1) .Basin side slopes above the permanent pool shall have a run to rise ratio <br />of 4:1 or flatter. <br />